Side by side menu with collapse sub menu

I’m using Ionic and flexbox for the first time and I’m struggling to organize my layout.

I’m trying to do a vertical menu where the user can select one topic. Each topic has his own subtopics. If one topic is selected a new row should appear with the related subtopics. The topics should be listed side by side.

The problem is… I’m using a row and col-50 to draw the topics side by side, and adding another row inside each col-50 with a col-100. The subtopics row becomes restricted by the parent col-50, not using the 100% width.

What’s the easiest way to solve this problem?